Don’t Retire a Pipe Based On Age Alone

When it comes to water systems, life expectancy and age alone are not accurate indications of pipe integrity. Many pipes have been found to have significant remaining service life even after the end of their theoretical design. Basing pipe replacement strictly on age can result in the unnecessary removal of healthy pipe, thus wasting money.

A more efficient approach for water utilities to determine whether a pipe needs to be replaced is acoustic-based pipe condition assessment. This method can help them better understand the levels of pipe degradation in their water systems. It can also help them to effectively prioritize replacement and rehabilitation projects according to remaining wall thickness levels, not age.

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The Project
The City of Columbus, Ohio, is significantly increasing its budget for large-scale pipe replacement projects. Malcolm Pirnie, a Division of Arcadis and the engineering firm that is assisting Columbus with its water infrastructure improvement projects, approached Toronto-based Echologics to assess the remaining wall thickness of selected 6- to 12-in. ductile iron and cast iron water pipes that run throughout the city using its acoustic-based condition assessment method — known as “Pipe Integrity Testing (PIT).” The results would be compared to samples of the same pipes, which would be collected after the assessment was complete.

The Method: Pipe Integrity Testing with Acoustics
Pipe integrity testing with acoustics uses calculated measurements of the speed of acoustical signals in the pipe and can accurately measure the pipe’s remaining wall thickness with little to no service interruption. The speed of sound in a pipe is governed by the water hammer equation which is shown here in its simplified form:

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In order to acquire accurate speeds of sound, acoustic signals are generated in pipes by physically tapping on their fittings, flowing water from fire hydrants, or by using vibro-mechanical shakers to induce noise at specific frequencies. These signals are measured by acoustic sensors mounted on separate, easily accessible appurtenances, such as existing inline valves or on the pipes themselves.

Advanced technology measures the time it takes for the signals to reach each sensor, and the velocities are calculated using the time delays, as well as the space existing between the sensors. Using this method, the pipes remain pressurized while the measurements are calculated, thereby preventing service disruption.
Remaining wall thickness can be calculated by accurately measuring the velocity and using the other known variables to solve the appropriate version of Equation 1.

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Occasionally, however, dimensional or material data is not available and poses a problem. For example: Cast iron must be identified as either spun cast or pit cast, as each classification has a different modulus of elasticity, and sometimes the pipe’s manufacturer or installation date is unknown. Equation 1 can no longer be solved if some of these variables are unknown. In such circumstances, the absolute thickness cannot be provided. However, the pipe’s properties can be reasonably assumed based on historical data and the measured pipes can then be ranked by their comparative thickness. In this type of scenario, utilities can receive valuable data outlining the comparative degradation of the pipes.

Composite pipes, such as ductile iron pipe with concrete lining, require additional consideration in this calculation. Equation 1 will not indicate whether the thickness is lost in the pipe or the lining. The pipe should be treated as a composite structure and the concept of effective thickness should be used to account for the lining and composites. The concept of effective thickness is straightforward; because the lining and the host material provides stiffness to the pipe, the effective thickness of the lining can be calculated. That is to say, if the lining was removed, how much host material must be added to return the pipe to the original stiffness? This extra material is the effective thickness of the lining.

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The pipe integrity testing method gives an average value of minimum wall thickness over the length of the pipe. Therefore, the method can recognize minor levels of uniform corrosion and major levels of isolated pitting corrosion, but a pipe with high levels of isolated degradation will appear to have minor loss overall.

Any of the following descriptions would hold true for a pipe with 10 percent measured loss:
1. Uniform loss of 10 percent throughout the entire length around the entire circumference
2. Uniform loss of 20 percent throughout half of the length around the entire circumference
3. Uniform loss of 10 percent throughout the top half of the entire length of pipe

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For simple pipes that consist of one material, the average remaining wall thickness can be provided. For pipes of unknown origin or multiple materials, the surveyed pipes can be ranked from the healthiest to the most degraded.

The Results
Over the course of three days, Echologics used the acoustic-based pipe integrity testing method to survey 5.3 km (3.33 miles) of pipe throughout the City of Columbus, using the existing pipe fittings — without any disruption to service. Each site was divided into multiple sections, roughly 150 m (500 ft) in length.
The City contracted the Corrosion Probe Institute (CPI) to verify the data. Samples from each section were sent to CPI for independent analysis. CPI measured the pitting and remaining wall thickness for each tested section. Echologics’ and CPI’s results are shown in Figure 1:

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Figure 1 shows Echologics’ measured thickness for each site (green); CPI’s measured thickness for each corresponding coupon sample (orange); and, areas where measurement data from each company matched (blue).

As illustrated in the chart, acoustically-collected measurements of remaining pipe wall thickness matched the measurements collected by CPI for seven of the 10 sites. Acoustically measured thickness change and CPI’s data were within 10 percent to 20 percent on Sites 1, 2 and 10. Figure 2 compares the thickness loss predicted by Echologics with the thickness loss measured from the samples.

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CPI’s results show that the condition assessment performed by Echologics proved to be useful and accurate. It is expected that if more samples were taken, data from the samples would converge with Echologics’ results, as the condition assessment method used gives an average value of minimum wall thickness over the length of the pipe. Coupon sampling is not an ideal method to verify the results of this type of testing, since sampling looks at discrete locations, and the coupons may not be indicative of the degradation over the length of the pipe. Unfortunately, exhuming a statistically significant number of coupons is cost-prohibitive.

The City was satisfied with the level of correlation between the measurements provided by Echologics and the thickness of the exhumed samples.

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Marc Bracken is vice president and general manager of Toronto-based Echologics.

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